text: The sub-surface drainage can be accomplished through drains or pumped wells.
  The subsurface drains, if any, are characterized by drain depth and drainage capacity. The drains are located in the transition zone. The subsurface drainage facility can be applied to natural or artificial drainage systems. The functioning of an artificial drainage system can be regulated through a drainage control factor.
  By installing a drainage system with zero capacity one obtains the opportunity to have separate water and salt balances in the transition above and below drain level.
  The pumped wells, if any, are located in the aquifer. Their functioning is characterized by the well discharge.
  The drain and well water can be used for irrigation through a (re)use factor. This may affect the water and salt balance and on the irrigation efficiency or sufficiency.